In recent years, polyphenols, which have skin whitening effects and antioxidative effects, have attracted attention. For example, gallic acid and its esters have been expected to be applied as skin whitening agents or antioxitdants in many fields, for example, external preparations for skins such as cream and milky lotions; and hair treatment agents such as hair setting agents and hair styling gel. In addition, the distinctive effect on hair properties, that is, the effect of imparting elasticity to hair has been disclosed in Japanese Unexamined Patent Application Publication No. 2000-314084.
However, such a polyphenol exhibits disadvantageous properties in that it is an irritant and causes sensitization. Furthermore, the polyphenols readily color. For example, the use of gallic acid derivatives in a formulation leads to coloration or precipitation; hence, it is difficult to sufficiently exhibit skin whitening and antioxidative effects. To prevent such coloration or precipitation of the polyphenols, a process of adding a metalloporphyrin complex and an organic reducing agent is disclosed in Japanese Unexamined Patent Application Publication No. 63-145213, and a process of adding a specific polyol alone or with an antioxidant such as ascorbic acid is disclosed in Japanese Unexamined Patent Application Publication No. 6-239716. However, these processes are still unsatisfactory.
To solve the above-described problems on such polyphenols, processes for using glycosides have been known for a long time. For example, highly toxic hydroquinone is linked with glucose to produce arbutin having a skin whitening effect. A process for using glycoside in which a sugar derivative is linked with at least one hydroxyl group among three hydroxyl groups of gallic acid is disclosed in Japanese Unexamined Patent Application Publication No. 2000-319116. This patent discloses the following process: Gallic acid or its ester reacts with a sugar having partially or fully acetylated hydroxyl groups or with a sugar halogenized at an anomeric position in a solvent to form a glycoside in the presence of an acid catalyst, for example, boron trifluoride diethyl etherate (BF3.Et2O), tin tetrachloride (SnCl4), or zinc chloride (ZnCl2). The resulting glycoside is deprotected, if necessary in the presence of an acid catalyst or an alkaline catalyst, then extracted and purified by column chromatography.
However, the yield achieved by the process disclosed in Japanese Unexamined Patent Application Publication No. 2000-319116 is low; hence, the process is also unsatisfactory.
Japanese Unexamined Patent Application Publication Nos. 62-263194, 62-263195, 1-249796, and 5-51394 disclose the following: A compound having one or two phenolic hydroxyl group(s) in its molecule reacts with a peracetylated sugar derivative, for example, peracetylated glucose, while removing acetic acid formed as a by-product from the reaction system by distillation, to form a compound having a glycosidic bond formed by the elimination of acetic acid generated by allowing one of the phenolic hydroxyl groups of the compound having the phenolic hydroxyl groups in its molecule to react with an acetyl group attached via an oxygen atom to an anomeric carbon atom of the peracetylated sugar derivative. However, hitherto, a process for preparing a compound having at least two glycosidic bonds formed by the elimination of acetic acid generated by allowing at least two phenolic hydroxyl groups of a compound having at least two phenolic hydroxyl groups in its molecule to react with acetyl groups each linked via an oxygen atom to an anomeric carbon atom of a peracetylated sugar derivative, for example, peracetylated glucose, and a process for preparing a glycoside with the resulting compound, have not been reported.
It is an object of the present invention to provide a method for preparing a glycoside in which at least two phenolic hydroxyl groups of a compound having at least two phenolic hydroxyl groups in its molecule are glycosidically linked to at least two sugar molecules by allowing a compound having at least two phenolic hydroxyl groups in its molecule to react with at least two sugar molecules each having an acetylated hydroxyl group attached to an anomeric carbon atom.